Bistable circuits, generally known as flip-flop circuits, have been present in the prior art since the days of the vacuum tube and relay. The flip-flop circuits have two stable states and two input terminals or types of input signals, each of which corresponds with one of the two states. The circuit remains in either state until called upon to change to the other state by the application of the corresponding signal. The conventional flip-flop circuit uses two active devices with positive feedback in such a way that the two devices tend towards opposite states, one "off" and one "on."
The flip-flop or bistable multivibrator is characterized by its ability to maintain either of two possible states. It is widely used in counting circuits, shift registers, and memory circuits. FIG. 1 shows a conventional flip-flop circuit which employs active FET devices 2 and 4 which have their gates cross-coupled to the corresponding nodes 6 and 8, which are, in turn, connected through load devices 10 and 12 to the drain potential VDD. FET device 2 is turned on by a positive potential on the node 6 and opens the current path through the resistive load 10 between VDD and ground, causing the node 8 potential to drop. Since node 8 is connected to the gate of FET 4, FET 4 is turned off, stopping the current flow through the load device 12, thereby causing the potential of node 6 to rise. The rise in the potential of node 6, which is transferred to the gate of FET device 2 sustains the on-state of FET device 2, thereby making that state stable. The state of the flip-flop circuit of FIG. 1 is reversed by placing an "on" signal at node 8 which turns FET device 4 on, increasing the current flow through the load device 12, thereby reducing the potential at node 6, which in turn, reduces the potential on the gate of FET device 2, thereby turning it off. With FET device 2 off, current through the load device 10 is stopped and node 8 rises, causing the gate of FET device 4 to turn that device on, thereby sustaining the "on" state of FET device 4, so that the flip-flop circuit of FIG. 1 remains in its second stable state.
Many types of load devices 10 and 12 have been employed, such as simple diffusion resistors, self biased enhancement mode or depletion mode FET devices, and layers of polycrystalline silicon material lying on top of the passivation layer for the integrated circuit. Since flip-flop circuits, such as shown in FIG. 1, are used in large quantities for information storage applications, the area occupied by that circuit on the LSI chip strongly governs the information storage density which can be achieved. Any net reduction which can be achieved in the packing density for flip-flop circuitry will yield a direct economic benefit in the quantity of information which can be stored on an LSI chip for a particular application.